Full length articleEffects of increased alloying element content on NiAl-type precipitate formation, loading rate sensitivity, and ductility of Cu- and NiAl-precipitation-strengthened ferritic steels
Graphical abstract
CF-9 has a higher amount of alloying elements (Cu, Mn, Ni, and Al) than CF-2. More Mn is incorporated into NiAl, displacing Al. This substitution reduces the lattice misfit with the Fe matrix and hence nucleation barrier, resulting in more NiAl-type precipitates in CF-9. This increases the loading rate sensitivity in CF-9 because the precipitates impede the motion of edge dislocations. Additionally, Cu segregation is observed by in situ Auger studies which partially explains the lower ductility of CF-9.
Introduction
Some years ago, we developed a series of precipitation-strengthened steels that can be considered to be analogs of HSLA-100 without Cr and Mo, with simplified thermal processing (hot rolling followed by air cooling). One version has 70 ksi yield strength (480 MPa) and received the ASTM A710 grade B designation and has been used recently for the construction of two bridges in Illinois [1]. The strengthening is primarily derived from the presence of semi-coherent Cu-containing precipitates. Another version, designated NUCu-140, has 140 ksi yield strength (965 MPa), which is achieved by addition of Ni and Al to promote the formation of NiAl precipitates [2]. Subsequently, we systematically increased the concentration of principal alloying elements (Mn, Cu, Ni and Al) with the intent to achieve higher precipitate densities and volume fractions in a series of bcc-Cu and B2–NiAl-type precipitation-strengthened ferritic steels, designated as CF steels [3]. The yield strength of CF steels was found to increase with the amount of principal alloying elements, reaching yield strength of 232 ksi (∼1600 MPa) with 12.4 at. % alloying elements. Most of these CF steels have room-temperature ductility in the range of 11–30%, higher than other commercially available steels with comparable strength [1], [2], [3].
The excellent ductility of these CF steels was explained by the interaction of the screw dislocations in the ferritic matrix with semi-coherent Cu-containing or -alloyed precipitates. It is hypothesized that the strain field from a semi-coherent precipitate promotes the formation of a double kink in a nearby screw dislocation [1], [4], [5], [6]. The kink segments have the characteristics of an edge dislocation, which is quite mobile in bcc iron, thus resulting in improved ductility and toughness. The formation of bcc Cu-alloyed precipitates in binary Fe–Cu is well documented and has recently been established in heavily alloyed steels [3], [7], [8], [9], [10], [11], [12], [13]. The presence of B2–NiAl-type of precipitates forming on these Cu-alloyed precipitates has also been reported in the heavily alloyed steels after aging in 500–550 °C temperature range [3], [13], [14].
In this paper, we will report investigations of two outstanding issues related to these heavily Cu-alloyed CF steels, containing over 12 at. % of principal alloying elements (Mn, Cu, Ni and Al): strain-rate sensitivity of flow stress and copper segregation. First, our previous work demonstrated the monotonic increase of yield strength with the amount of alloying elements due to the co-precipitation of Cu-alloyed and B2–NiAl precipitates. As indicated in the preceding paragraph, our hypothesis is that these semi-coherent precipitates provide athermal activation of screw dislocation motion, thereby slowing increase of flow stress with decreasing temperatures. Since lower temperature can be simulated at ambient temperature by testing at higher strain or loading rates, we tested this hypothesis by measuring the flow stress of lightly Cu-alloyed steels (containing ≈ 6 at. % of principal alloying elements) as a function of strain rate and comparing the data with those from HSLA-65, which does not contain these semi-coherent precipitates [15]. The result demonstrated that lightly Cu-alloyed steels have significantly lower strain-rate sensitivity. We would like to extend this investigation to heavily Cu-alloyed CF steels.
Second, while the addition of Ni and Al to form NiAl precipitates is beneficial for enhanced strength, the removal of Ni from the matrix due to precipitation may facilitate the segregation of Cu to grain boundaries – one requires a certain minimum amount of Ni in the matrix to prevent Cu segregation [16], [17]. Further, typical aging temperatures of 500–550 °C used in our study fortuitously fall in the temperature range often observed to result in elemental segregation to grain boundaries [18], [19]. Experimental results and recent first-principles calculations along with computer simulations also show that Cu segregation to grain boundaries, driven by a decrease in the grain boundary energy, will have embrittling effects [18], [20], [21], [22], [23], [24]. Specifically, Yuasa et al. calculated that a Cu-segregated Fe sigma3 grain boundary will have a 27% lower elongation-to-failure than a clean one [20]. Interestingly, their calculation also predicted that this segregation did not have any impact on flow stress. Additionally, a higher amount of Al in the matrix has been consistently shown to reduce the ductility of ferritic steels [25], [26], [27] by lowering the dislocation mobility. Therefore, we would like to investigate the effect of the addition of (Ni + Al) on the ductility of CF steels.
Section snippets
Experimental methods
Compositions of two alloys used in this study are presented in Table 1. CF-2 was arc-melted and cold-swaged from a diameter of 20 mm–8 mm. CF-9 was produced by Sophisticated Alloys as a 22.5 kg ingot and hot-rolled. The composition was determined by spectrographic analysis. Both alloys were solution-treated at 950 °C followed by water quenching and aged at 500 °C and 550 °C. In other experiments, CF-9 was solution-treated at 950 °C followed by aging in the temperature range of 400 °C–600 °C. A
Aging studies and atom probe tomography
Fig. 1 shows the evolution of Vickers hardness for CF-2 as a function of time at 500 °C and 550 °C. The highest hardness, 380 VHN, was achieved at 500 °C after aging for 2 h. The difference in hardness between 500 °C and 550 °C can be explained by the difference in precipitated volume fraction at the respective temperatures. Based on the solubilities at 500 °C and 550 °C, it is reasonable to assume that the precipitated volume fraction at 500 °C will be higher than that at 550 °C. The peak
Conclusions
In this study, we explored two experimental Cu- and NiAl-precipitation-strengthened ferritic steels, one being more heavily alloyed with Mn, Cu, Ni, and Al as principal alloying elements. Both the volume fraction and number density of NiAl-type precipitates are higher in the heavier alloyed steel (designated as CF-9) than the lighter alloyed steel (designated as CF-2), by a factor of 70 and 60 times respectively, much more than the ratio of the total concentration of principal alloying
Acknowledgments
This work was supported by the National Science Foundation, Grant No. CMMI-0826535 and made use of Northwestern University's Optical Microscopy and Metallographic Facility and the Center for Atom Probe Tomography, supported by the MRSEC program of the National Science Foundation, Grant No. DMR-1121262. The LEAP tomograph at NUCAPT was purchased and upgraded with funding from NSF-MRI (DMR-0420532) and ONR-DURIP (N00014–0400798, N00014–0610539, N00014-0910781) grants. Additional instrumentation
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She presently works at Structural Materials Development Division at National Energy Technology Lab as an ORISE post doctoral fellow.